In view of their nature, mobile terminals such as portable phones and personal digital assistants (PDAs) are powered by rechargeable batteries. To charge the batteries, the mobile terminals apply electric energy to the batteries through separate chargers. Typically, the charger and the battery each have an exterior contact terminal and thus are electrically connected to each other by contacting their contact terminals.
This contact-based charging scheme faces the problem of vulnerability of contact terminals to contamination of foreign materials and the resulting unreliable battery charging because the contact terminals protrude outward. Moreover, if the contact terminals are exposed to moisture, the batteries are not charged properly.
To address the above problem, wireless charging or contactless charging technologies have recently been developed and applied to many electronic devices.
Such a wireless charging technology is based on wireless power transmission and reception. For example, once a portable phone is placed on a charging pad without being connected to an additional charging connector, its battery is automatically charged. Among wirelessly charged products, wireless electric toothbrushes or wireless electric shavers are well known. The wireless charging technology offers the benefits of increased waterproofness due to wireless charging of electronic products and enhanced portability due to no need for a wired charger for electronic devices. Further, it is expected that various relevant wireless charging technologies will be more developed in the upcoming era of electric vehicles.
There are largely three wireless charging schemes, namely electromagnetic induction using coils, resonance-based, and radio frequency (RF)/microwave radiation based on conversion of electric energy to microwaves.
So far, the electromagnetic induction-based wireless charging scheme has been dominantly popular. However, considering recent successful experiments in wireless power transmission over microwaves at a distance of tens of meters in Korea and other overseas countries, it is foreseeable that every electronic product will be charged cordlessly at any time in any place in the near future.
Electromagnetic induction-based power transmission means power transfer between primary and secondary coils. When a magnet moves through a coil, current is induced. Based on this principle, a transmitter creates a magnetic field and a receiver produces energy by current induced by a change in the magnetic field. This phenomenon is called magnetic induction, and power transmission based on magnetic induction is highly efficient in energy transfer.
In 2005, regarding resonance-based wireless charging, professor Soljacic in the Massachusetts Institute of Technology (MIT) suggested a system that makes wireless energy transfer from a charger at a distance of a few meters based on the resonance-based power transmission principle by the coupled mode theory. The wireless charging system of the MIT team is based on a physics concept called resonance in which a sounding tuning fork causes a nearby wine glass to chime at the same frequency. The MIT team resonated electromagnetic waves carrying electric energy, instead of sound. The resonant electric energy is directly transferred only in the presence of a device having the same resonant frequency, while the unused electric energy is reabsorbed into the electromagnetic field rather than it is dispersed in the air. Thus the resonant electric energy does not affect nearby machines or human bodies, compared to other electronic waves.
The charging efficiency of a wireless power receiver depends on impedance change. For example, each of a wireless power transmitter and a wireless power receiver may perform impedance matching. If impedance is matched, charging efficiency may be increased relatively. Meanwhile, the wireless power receiver may include a matching circuit which has electric devices such as a capacitor or an inductor, for impedance matching. The wireless power receiver may perform impedance matching by operating the matching circuit, thus increasing charging efficiency. However, since the matching circuit includes electric devices, for impedance matching, it increases the weight and volume of an electronic device.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.